Tech Info

The following paper by one of the greatest, most prolific audio authors of all time (and my personal hero), Norman H. Crowhurst, details the earliest reference to blocking distortion solutions I have seen: Puzzled About Amplifiers? by Norman Crowhurst, Audio, November 1959 Puzzled About Amplifiers? - (.pdf format)

For those of you still confused about how to determine if an amplifier is "class A" or "class AB", here are the definitions from the RCA receiving tube manual. Note the highlighted sections, in particular, the first section where it says: "This classification depends primarily on the fraction of input cycle during which plate current is expected to flow under rated full-load conditions". This is not an ambiguous thing, as some would have you believe: Amplifier Classes(.pdf format)

And, for those of you who still think class AB amplifiers are "class A at lower volumes", read the difference in this next paper, which agrees with the RCA receiving tube manual above:

The following paper illustrates both the design of shunt RC networks across the output transformer primary and the use of current feedback to modify the damping factor of an amp: Zero-Impedance Output Stage: by Raymond G. Anthes, Audio Engineering, January 1953: Zero-Impedance Output Stage (.pdf format)